The prior art devices in this general field include those in which infrared radiation is caused to pass through separate paths, through a comparison gas and a gas to be analyzed so that the relative absorption characteristics of the two gases can be determined. In German Pat. No. 730,478, a device of this general type is shown in which in each ray path equal receiving layers are disposed among themselves and the differences of the radiation energy absorbed in the receiving layers is measured as a measure of the concentration of the gas component which is being determined. In the case of this device, it is a disadvantage that the rear walls of the receiving layers are made of a material which is impermeable to radiation so that the radiation is reflected from those rear walls and proportionately once more passes through the receiving layers and the cuvettes and a non inconsiderable portion of this radiation again reaches the radiator unit. From there, a part of the radiation is again reflected back through the cuvettes into the measuring chamber. As a consequence, ever larger portions of the marginal zones of the absorption areas are stimulated and corresponding transverse responses for interfering gases develop. The path from the radiator unit to the receiving layers and back may then be traversed several times, depending upon the absorption conditions in the intermediate gas layers and windows. All changes in the absorption and deflection behavior in the construction components of this device, for example, surface influences as a result of changes in temperature, precipitations of moisture, chemical reactions, etc., are therefore contributing factors to instabilities of the zero point of the device and of the sensitivity thereof.
Another device is shown in German Pat. No. 976,290, wherein the rear wall of the receiving layers consists of a raw material permeable to the radiation. This is an effort to reduce the reflection, but does not, by itself, eliminate the above-described deficiencies because, on the one hand, radiation emanating from the source of radiation is reflected again as a result of the rear window with the above-described consequences. On the other hand, radiation from the surroundings may penetrate, for example, from outside sources of heat, which emit radiation having portions in the spectral range of interest. As a result of changes in the environment, the residual radiation entering through the rear window into the receiving layers is undefined and not constant, as a result of which insufficient stability of the device results.
In the case of many measures which have been attempted for improvement in nondispersive infrared gas analysis devices, the achievement of a stable zero point has been regarded as a preeminent requirement. Zero point stability was sought by the arrangement of several receiving chambers acted upon by the radiation, by special forming or shaping of the receiving chambers, etc., and the measures taken in these devices led to the construction of more complicated apparatus which is more subject to breakdowns. On the other hand, mechanisms have been known to aid in the establishment and control of the zero point, including automatic regulation at certain time intervals, either fixed or adjustable, so that the improving measures previously mentioned for the IR analysis devices themselves are no longer decisive for the achievement of a stable zero point.